The present invention concerns the tread of a road tire having a radial carcass reinforcement and a tread reinforcement and more particularly intended for traveling on snow-covered, icy or wet ground.
Such a tread is generally formed of elements in relief (ribs or blocks), separated from each other in circumferential direction and/or in transverse direction by grooves and provided with numerous incisions or slits the width of which is other than zero but far less than the width of the circumferential and transverse grooves which define the elements in relief. The width of an incision, depending on the size of tire in question, is generally between 0.1 mm and 2 mm.
French Patent No. 2,461,602 describes the means for imparting to the tire better longitudinal adherence on the types of ground in question, both under drive torque and under braking torque. These means consist in providing the elements in relief of the lateral zones of the tread with incisions of substantially transverse orientation on the tread but inclined with respect to the normal to the surface of the tread by an angle of at most 45.degree. in the direction of rotation of the tire, and in providing the elements in relief of the central zone with incisions inclined in the direction opposite the direction of rotation. Thus the incisions of the lateral elements in relief increase adherence during the acceleration while the incisions of the elements in relief of the central zone increase adherence during the braking.
It is known to the man skilled in the art that, in general, any feature the purpose of which is to improve the adherence of a tire causes a decrease of the resistance to wear. This is the case with the inclination of the incisions since the improvement obtained with respect to adherence to the ground in question is the consequence of the formation of abnormal wear of the strips of rubber defined by the facing portions of two successive incisions which are substantially parallel to each other. Depending on the orientation of the incision with respect to the direction of rotation of the tire, this wear makes it possible to preserve effective edges of the strips of rubber.
This type of wear, also known as irregular wear, is not however without drawbacks. It detrimentally affects not only the strips of rubber contained between two adjacent incisions but also the element in relief or block itself, in particular as a result of the considerable variation in the distances between the incisions closest to the grooves defining the block and the walls of the grooves.
Furthermore, as the incisions are inclined with respect to the normal to the surface of the tread at angles of opposite sign and not necessarily equal in absolute value, depending on whether they are located at the edge of the tread or at the center thereof, the irregular wear which detrimentally affects the elements in relief differs as a function of the axial position thereof on the tread. The same is true in respect of the rate of wear of these blocks which causes the surface of the tread to be devoid of uniformity, thus detrimentally affecting the wear life of the tire and the effectiveness of the incisions as from a certain rate of wear.